1. Technical Field of the Invention
The present invention relates to dental compositions. More precisely, the dental compositions according to the present invention are useful for producing dental prostheses and for dental restoration.
2. Description of Background and/or Related and/or Prior Art
The conventional dental compositions are typically epoxy resins or photopolymerizable silicones or free-radically polymerizable acrylate resins. These compositions further include particulate reinforcing fillers (e.g., of hydrophobicized silica), photoinitiators and optionally photosensitizers in the case of cationic compositions or free-radical initiators for the free-radical compositions, and indeed other functional additives such as pigments or stabilizers.
After they have been mixed, these compositions are shaped and then photocrosslinked to a mass whose structure is like that of the teeth.
The fact that the filler is composed of very fine particles (≈0.05 μm) with a high specific surface area is a factor which limits its degree of incorporation into the resin. This is because the absorption capacity of the resin is limited. The result is that the filler levels of such compositions rarely reach more than 45% by volume.
This is therefore to the detriment of the mechanical reinforcement function assigned to the particulate filler.
This particulate filler may also interact with the reactive functions of the (photo)polymerizable/crosslinkable species and may therefore underlie problems of instability of the dental composition.
U.S. Pat. No. 6,306,926-B relates to dental compositions based on epoxy (e.g., polyTetraHydroFuran, UVR® 6105, EPON® 828, GY281®), oxetane or vinyl ether resins, among others, which are polymerizable/crosslinkable, cationically and under irradiation, and optionally (meth)acrylate resins which are polymerizable free-radically. In addition to the polymerization initiators, which are cationic photoinitiators and optionally free-radical initiators, as appropriate, these compositions comprise a microparticulate mineral filler which is radioopaque and is selected from the following metal compounds: oxides, halides, borates, phosphates, silicates, carbonates, germanates, tetrafluoroborates and hexafluorophosphates, having an isoelectric point of less than 7. This composition is such that its Barcol hardness is at least 10 after 30 minutes of cationic polymerization at 25° C.
These resins have the drawback of not being perfectly transparent to the actinic activating radiation of the UV-visible actinic polymerization, which is detrimental to the reaction kinetics and hence limits the possibilities of obtaining very thick photocrosslinked materials.
FR-A-2,784,025 is aimed at remedying this by proposing dental compositions based on silicone resins which are polymerizable/crosslinkable, cationically and under irradiation, with or without subsequent thermal post-crosslinking. These silicone resins contain oxirane (epoxide, oxetane, etc.) or vinyl ether functionalities, etc. Such compositions comprise one or more cationically polymerizable and/or crosslinkable polydimethylsiloxanes which carry on at least one of their ends reactive functions of formula:
an effective amount of at least one onium borate initiator:
at least one photosensitizer, and an inert reinforcing or dental filler based on polymethyl methacrylate or on hexamethyldisilazane-treated fumed silica with a specific surface area of 200 m2/g, which is present in the composition in a proportion of at least 10% by weight relative to the total weight of the composition. These dental compositions are intended for the manufacture of prostheses or dental devices.
These silicones have the advantage over organic resins which crosslink cationically of being highly transparent to UV-visible light and hence of allowing very thick materials (several millimeters thick) to be obtained which are photocrosslinked within a very short time (less than one minute) with a UV lamp which emits in the visible field >400 nm.
These silicones, however, are formulated with reinforcing fillers that have Lewis or Brønsted acid character, such as quartzes or fumed silicas of very low particle size, whose surface silanols and/or residual water react with the cationic functions. Silicone formulations of this kind are therefore unstable on storage of the compositions.
Besides this problem of instability, brought about by the fillers, these silicone dental compositions remain capable of improvement in terms of increasing the degree of filling, optionally substituted so as to allow improvement in the mechanical properties.
Also known, through EP-A-1,050,291, are highly charged dental compositions which are presented as being endowed with good mechanical properties and contain from 10 to 70% by volume of filler (e.g., fumed silica) with a particle size Φm of 0.05-0.5 μm (less than 50% by volume of particles with a diameter Φ>0.50 μm), a free-radically photopolymerizable acrylic monomer and a phosphoric ester dispersant of formula:R-[—CO—(CH2)5—O—]n-PO3H2.
Teaching of this kind relating to free-radical dental compositions can in no way be transposed to cationic dental compositions based on silicone. This is because the R-[—CO—(CH2)5—O—]n-PO3H2 dispersants are not suitable for the cationic compositions, in particular since they contain a significant acid residue, RPO3H2, which reacts in the presence of oxirane functions and is detrimental to the stability of the composition.
It is therefore apparent that the prior art provides no satisfactory solution to the twin problem of stabilizing dental compositions based on units which are polymerizable cationically under UV (oxiranes, for example) and dispersing sizable amounts of fillers in the resin.